NEW YORK  If a time traveler from a hundred years ago were to visit ahome today, much of the technology would be completely alien. Thetelevision, cordless phone and computer would probably leave himflabbergasted.

But on seeing a light bulb, he might say, "Ah! Here's something Irecognize. A few of those grace my home, too."

If the visitor comes back in 15 years, the fruit of Thomas Edison'sbright idea may be gone. The likely replacement: light-emittingdiodes, or LEDs.

LED lamps were unthinkable until the technology cleared a major hurdlejust a dozen years ago. Since then, LEDs have evolved quickly and arebeing adapted for many uses, including pool illumination and readinglights, as evidenced at the Lightfair trade show here this week.

More widespread use could lead to big energy savings and a minorrevolution in the way we think about lighting.

LEDs have been around since the 60s, but have mostly been relegated toshowing the time in an alarm clock or the battery level of a videocamera.

They haven't been used as sources of illumination because they, for along time, could not produce white light — only red, green and yellow. Nichia Chemical of Japan changed that in 1993 when it startedproducing blue LEDs, which combined with red and green produce whitelight, opening up a whole new field for the technology.

And the industry has been quick to exploit it. LEDs are based onsemiconductor technology, just like computer processors, and areincreasing in brightness, energy efficiency and longevity in a waythat's reminiscent of the way each year's new crop of processors isfaster and cheaper than last year's.

Just this week, researchers at the Lighting Research Center atRensselaer Polytechnic Institute in Troy, N.Y., said they had boostedthe light output per watt of a white LED to almost six times that ofan incandescent light bulb, beating even a compact fluorescent bulb inefficiency.

The current generation of mass-produced white LEDs is not aseffective. It's about twice as good as a light bulb of the samewattage, but the energy savings aren't enough to overcome the majordrawback of being expensive.

"It's hard to convince consumers based on energy savings alone," saidNadarajah Narendran, director of lighting research at Rensselaer. "Ifyou look at compact fluorescent lamps, they're four times as efficientas incandescent lights, and how many homes have those? It's less than5 percent penetration."

But development is brisk, and the Department of Energy has estimatedthat LED lighting could cut national energy consumption for lightingby 29 percent by 2025. The total savings on U.S. household electricbills until then would be $125 billion.

LEDs have other advantages that are propelling them into niche uses,despite their upfront cost. Current white LEDs will last up to 50,000hours, about 50 times as long as a 60-watt bulb. That's almost sixyears if they're on constantly.

That makes them attractive for places where changing bulbs isdifficult or expensive — like on the outside of buildings or inswimming pools. Osram Sylvania, the lighting subsidiary of Germanmanufacturer Siemens AG, makes 27-foot long strips of flexible,adhesive tape covered in LEDs for such applications.

Hotels are interested in using LEDs in bedside lamps to save them thetrouble of replacing burned-out bulbs, said Jim Anderson of LaminaCeramics, which showed off a 6-watt array of LEDs that produce lightequivalent to a 20-watt halogen bulb.

LEDs are also durable. Being solid-state, they can resist thevibrations in aircraft and cars, according to Narendran, who hasworked with Boeing Co. on designs for aircraft cabins.

General Electric Co. and smaller iLight Technologies of Evanston,Ill., make glowing LED signs that look like neon. Neon lighting is aleading cause of fires at restaurants and the signs are vulnerable tovandalism.

By contrast, LED signs made of Plexiglas are tough. At the tradeshow, iLight exhibited an LED sign that still worked after taking ablast from a shotgun. The limitation: iLight's signs can't be madeeconomically on a one-off basis, as done at small neon-sign shopsaround the country.

The feature of LEDs likely to propel them into homes is aesthetic, notpractical. Arrays that mix red, green and blue LEDs can produce anycolor of the rainbow. Instead of a dimmer, you might have threesliding knobs that let you mix color.

"On a very hot day you might want blue light to cool it down a bit, oron a winter day you may want to simulate sunlight," said Steve Landauof Lumileds Lighting, an LED-making joint venture of AgilentTechnologies Inc. and Philips Lighting.

Qantas Airways Ltd., the Australian airline, recently outfitted itsfirst-class cabin with LED lighting that shines a deep blue when it'stime to sleep.

A system like that would be too expensive for most homes, but industryexperts believe the price will come down in a few years as thetechnology develops.

"We are still in a very young research environment," said NorbertHiller, vice president at Cree Inc. of Durham, N.C., which producesblue and green LEDs. "Our researchers keep surprising us."